SDF (soluble dietary fiber) and IDF (insoluble dietary fiber) fractions were extracted from 14 different foods by the modified Prosky method. Ion exchange chromatography with DEAE cellulose was employed to examine each of the fractionated sugars. Aloe fractions were further subjected to gel chromatography to examine the effect of heating on molecular weight. The SDF fractions of anhydrous samples of okra, cabbage, celery, bitter gourd, and carrot were large when autoclaved, while that of aloe was largest when unheated, and those of eggplant, edible burdock, Japanese radish, hijiki, and nameko were large when microwaved. The SDF fractions of moroheiya and okra included acid polysaccharides in large quantities, and acid polysaccharides increased further with autoclaving. The SDF fraction of celery did not show as pronounced a tendency for heat-induced increase of acid sugar. IR analysis confirmed that heating affected the functional groups of this fraction.Keywords: dietary fiber, autoclave, microwave, acid polysaccharides, IR analysisWe have heretofore focused on soya in our studies of the effect of heating on dietary fiber (DF), whereupon it has been pointed out that different foods are affected differently by heat processing, owing to differences in the DF each food contains (Takeyama et al., 1986(Takeyama et al., , 1991(Takeyama et al., , 1996. We therefore chose to study the effect of heating on the DF content and polysaccharide composition of SDF (water soluble DF) and IDF (water insoluble DF) fractions extracted by the modified Prosky method (Prosky et al., 1988) from 14 different samples.To study polysaccharide composition, we focused primarily on the SDF fractions of aloe, moroheiya, okra, and celery, tested by ion exchange chromatography and IR analysis, while aloe was subjected to further testing by gel filtration.
Materials and MethodsSamples Samples consisted of 14 different low fat, low protein foods (Table 1). Each sample was washed and wiped dry, whereupon the edible portion was either grated using a plastic or ceramic grater, or minced with a ceramic knife. A portion of each sample was either placed in a commercially available microwaveable package and intermittently heated in a microwave oven for 8 to 13 min, or placed in a tall beaker, covered by a petri dish, and autoclaved at 121˚C for 30 min. Each of the processed samples was then freeze dried together with the unheated samples, then pulverized and put through a 32-mesh sieve for use in our study. All equipment was soaked overnight in 15% nitric acid solution, then washed with tap water and purified water before use.Extraction of IDF and SDF fractions by the modified Prosky method Two to 6 g of the samples were subjected to extraction by the modified Prosky method (Prosky et al., 1988) as described in a previous report (Takeyama et al., 2001), followed by dialysis and freeze drying, to obtain IDF and SDF fractions. IDF was filtered by suction with a PYREX 17G3 glass filter and the SDF was filtered by suction using a HARIO 17G5 glass filter (T...